Aromatic hydrocarbons, notably benzene, toluene and ortho- and para-xylene (collectively, mixed xylenes), are important industrial commodities used, for example, to produce numerous chemicals, fibers, plastics, and polymers, including styrene, phenol, aniline, polyester, and nylon.
Mixtures of aromatic- and paraffinic hydrocarbons can be produced by converting alkanols in the presence of an oxygenate conversion catalyst, such as a zeolite catalyst. For example, methanol can be converted to gasoline range paraffins, aromatics, and olefins. Higher alcohols, such as ethanol, n-propanol, isopropanol, n-butanol, 2-butanol, isobutanol, tert-butyl alcohol, pentanol, and hexanol, can also be converted to hydrocarbons using this process.
When oxygenates are converted to hydrocarbons in the presence of a zeolite catalyst, the hydrogen-to-carbon effective ratio (H:Ceff ratio) of the reactants affects the H:Ceff ratio of the reaction products. The H:Ceff ratio is calculated as follows:
            H      :              C        eff              =                  H        -                  2          ⁢          O                    C        ,where H represents the number of hydrogen atoms, O represents the number of oxygen atoms, and C represents the number of carbon atoms. Water and molecular hydrogen (diatomic hydrogen, H2) are excluded from the calculation. The H:Ceff ratio applies both to individual components and to mixtures of components, but is not valid for components which contain atoms other than carbon, hydrogen, and oxygen. For mixtures, the C, H, and O are summed over all components exclusive of water and molecular hydrogen. The term “hydrogen” refers to any hydrogen atom while the term “molecular hydrogen” is limited to diatomic hydrogen, H2.For illustration purposes, the H:Ceff ratio of ethanol (and of all alkanols) is 2, as shown in Table 1 below.
TABLE 1H:Ceff Ratio of AlcoholsAlcohol (by number ofcarbon atoms)H:CeffC2C22C32C42C52C62C72C82C92↓↓C∞2
Paraffins generally have a H:Ceff ratio greater than 2, while alkyl mono-aromatic compounds generally have a H:Ceff ratio between 1 and 2, as shown in Tables 2 and 3 below.
TABLE 2H:Ceff Ratio of ParaffinsParaffinsH:CeffC14C23C32.67C42.5C52.4C62.33C72.29C82.25C92.22↓↓C∞2
TABLE 3H:Ceff Ratio of Alkyl Substituted Mono-AromaticsAromaticH:CeffBenzene1.0Toluene1.14Xylene1.25C91.33↓↓C∞2
Other species of interest include carbon dioxide (CO2) with a H:Ceff ratio of −4, carbon monoxide (CO) with a H:Ceff ratio of −2, and carbon (C) with a H:Ceff ratio of 0. Carbonaceous residue, or coke, that may accumulate on catalyst or other surfaces exhibits a range of H:Ceff ratios, depending on the amount of residual hydrogen and oxygen within the coke.
For the conversion of alkanols to hydrocarbons, many feeds of interest are essentially free of atoms other than C, H, and O, allowing from a practical standpoint the characterization of the feed to a reaction step using the H:Ceff ratio and the products of a reaction step using the H:Ceff ratio. For instance, alkanols can react across zeolite catalysts to form a mixture of hydrocarbons. Because of the high H:Ceff ratio of alkanols, conversion of alkanols across zeolite catalysts generally yields a relatively high ratio of paraffins to aromatics—approximately three moles of paraffins are generated per mole of benzene or alkyl-substituted mono-aromatics. This is a desirable mixture for some applications, such as gasoline production. However, the low yield of aromatics limits the application of this process for the production of high value aromatic chemicals such as benzene, toluene, and xylenes (BTX).
Zhang et al. recently studied the impact of the H:Ceff ratio on the conversion of biomass-derived feedstocks to coke, olefins and aromatics using a ZSM-5 catalyst (Zhang et al., Catalytic conversion of biomass-derived feedstocks into olefins and aromatics with ZSM-5: the hydrogen to carbon effective ratio, Energy Environ. Sci., 2011, 4, 2297). In this study, Zhang reported that biomass derived feedstocks having H:Ceff ratios of between 0 and 0.3 produced high levels of coke, making it uneconomical to convert biomass derived feedstocks to aromatics and chemicals. Zhang also reported that the aromatic+olefin yield increases and the coke yield decreases with increasing H:Ceff ratio of the feed. However, there is an inflection point at a H:Ceff ratio of 1.2, where the aromatic+olefin yield does not increase as rapidly. The ratio of olefins to aromatics also increases with increasing H:Ceff ratio, while CO and CO2 yields go through a maximum with increasing H:Ceff ratio. Specifically, Zhang reported that the aromatic and olefin yields increased from 12% and 15% to 24% and 56% with increasing H:Ceff ratio, respectively, and that the olefin yield is higher than the aromatic yield for all feedstocks, with the gap increasing with an increase of the H:Ceff ratio. Once again, this low yield of aromatics limits the application of the Zhang process for the production of high value aromatic chemicals such as benzene, toluene, and xylenes (BTX).
There remains a need for a method to increase the yield of aromatic hydrocarbons produced when converting alkanols to hydrocarbons.